A transmitted-light base serves to transilluminate an object in a manner suitable for viewing with a microscope. For that purpose, the light directed by the transmitted-light base onto the object generates a radiation at a solid angle at every point in the illuminated object field. The radiation flux that proceeds from a point in the object field at an infinitesimal solid angle is referred to as the radiant intensity.
“Regulation of the illumination intensity” of a transmitted-light base is understood here to be regulation of the radiant intensity. In contrast, for example, to modifications of the illuminated field diameter or the numerical aperture of the illumination system, which influence the distribution of the radiant intensity as a function of location or angle, in the context of a regulation of the illumination intensity of the transmitted-light base only the radiant intensity is regulated, independently of location and angle.
Transmitted-light bases of this kind having an integrated light source are known. The light source usually has associated with it an electrical power regulation system with which the current or voltage at the light source can be adjusted. Regulation of the illumination intensity is then usually accomplished by varying the voltage at the light source. Provision may furthermore be made, alternatively or additionally, for the use of neutral gray filters.
A regulation of the illumination intensity may be necessary for various reasons: on the one hand, known transmitted-light bases allow object illumination using various illumination modes, e.g., transmitted bright-field, oblique illumination, or relief contrast. These illumination modes, like the object-dependent transmission properties of the specimens being examined, greatly influence the image brightness. Ultimately the desired image brightness also depends on the individual user. A regulation of the illumination intensity is also necessary during an object examination using a zoom microscope, since the image-side aperture and therefore the image brightness vary upon actuation of the zoom. An object examination at constant image brightness is, however, necessary or at least desirable, for example in order to avoid incorrect exposures when cameras are used, or to ensure the comparability of images of an examination series.
The aforementioned factors that can influence image brightness show that a high dynamic range in the illumination intensity is necessary in order to be able to use all observation or illumination modes and device configurations.
For specific applications, moreover, not only a specific image brightness but also a specific color temperature or spectral intensity distribution is necessary. “Color temperature” refers to the temperature of the black body having (approximately) the same spectral distribution as the illumination source. In general, the color temperature of electrical illumination sources changes when the electrical power level delivered to the light source changes. In the case of incandescent or halogen lamps, which have light emission characteristics similar to a black body, the color temperature of the spectrum emitted by the light source shifts, upon a reduction in the electrical power level delivered, from the blue spectral region to the red spectral region (also referred to as “red shift”). A color temperature shift of this kind modifies the perceived color of the object image, thus making it difficult to compare the results in different illumination modes. This illustrates the fact that for specific applications, not only an (almost) constant image brightness but also an (almost) constant color temperature is desired.
EP 1 418 454 A2 discloses a microscope and a method with which the brightness and the perceived color of the object image can be kept constant. The teaching of this document is based on the problem that changes in settings regarding the resolution and contrast of the microscope are overlain by a change in the brightness of the microscope image. At the same time, with ordinary light sources a brightness correction results (as already mentioned above) in a change in color temperature. The aforesaid document proposes, as a solution, to arrange in the illumination or image beam path spectral correction means that correct the change caused in the spectral intensity distribution (color temperature) of the light emitted by the light source, in such a way that the spectral intensity distribution of the light directed onto the object remains largely unchanged. The spectral correction means encompasses a color filter that is embodied as a circular disk-shaped interference filter, different filter areas exhibiting different spectral interference capabilities and associated transmission capabilities. Depending on the rotation of the circular disk-shaped filter, a color temperature decrease occurring as a result of a voltage decrease can be compensated for by partial introduction of the corresponding filter area into the aperture of the illumination device. Further suitable spectral correction means encompass absorption or reflection filters. In an automated method, for example, the aperture of the illumination beam path is decreased in order to increase the image contrast, this being associated on the other hand with lower resolution and lower image brightness. The decreased image brightness can automatically be compensated for by means of a control computer, by way of an increase in the electrical power level to be delivered to the light source of the transmitted-light base. At the same time, the control computer also calculates the requisite position of the color filter, so that an almost unchanged spectral intensity distribution of the light directed onto the object exists.
The teaching described in EP 1 418 454 A2 is usable for compound microscopes, and requires that an aperture device of the illumination system (e.g., diaphragm) be accessible. In this context, the aperture and the image of the objective entrance pupil generated by the illumination condenser correspond to one another. In zoom microscopes having a high zoom factor, the objective entrance pupil corresponds to an aperture that changes in terms of diameter and location upon actuation of the zoom. In microscopes having a high zoom factor in particular, the aperture—and therefore the filter area serving to correct the color temperature—changes appreciably. In stereo zoom microscopes, the two entrance pupils correspond to two apertures, one for the right and one for the left stereoscopic beam path, which change in terms of both their diameter and their location upon actuation of the zoom. It cannot be expected that introduction of a color filter into the illumination beam path, as proposed in the aforesaid document, will result upon zooming in a constant color temperature that is identical for both stereo channels.
Transmitted-light bases without an integrated light source are fed by light guides, associated with which are lamp housings that permit both controllability of the image brightness at a constant color temperature and the setting of a specified color temperature. This is done by way of a mechanical arc-shaped stop and a voltage regulation system in the lamp housing. A device of this kind as known from lamp housings for fiber illumination systems is not, however, implemented in transmitted-light bases having an integrated illumination system. In these lamp housings, the light of a reflector lamp is concentrated onto the small diameter of the light guide entrance and there cut off, as applicable, by an arc-shaped stop. The inhomogeneity of the illumination of the fiber entrance is almost eliminated by the intermixing of the fibers in the light guide, so that the light guide exit can be used for homogeneous illumination of an object. This advantageous arrangement in lamp housings necessitates the use of light guides, and therefore cannot be used in transmitted-light bases having an integrated light source.